Improve Makefile

[itchyny]

• Add -w -s in make cross too.
• Move phony targets (ref: Clark Grubb's Makefile style guide)
• := is preferred than = in performance (especially when referred multiple times)

Before

~/.go/src/github.com/mackerelio/mkr ls -lh snapshot/
total 66824
-rw-r--r--  1 itchyny  staff   7.5M Mar 20 17:42 mkr_darwin_386.zip
-rw-r--r--  1 itchyny  staff   7.7M Mar 20 17:42 mkr_darwin_amd64.zip
-rw-r--r--  1 itchyny  staff   7.6M Mar 20 17:42 mkr_linux_386.tar.gz
-rw-r--r--  1 itchyny  staff   7.7M Mar 20 17:42 mkr_linux_amd64.tar.gz

After

~/.go/src/github.com/mackerelio/mkr ls -lh snapshot/
total 37936
-rw-r--r--  1 itchyny  staff   4.5M Mar 20 17:59 mkr_darwin_386.zip
-rw-r--r--  1 itchyny  staff   4.7M Mar 20 17:59 mkr_darwin_amd64.zip
-rw-r--r--  1 itchyny  staff   4.2M Mar 20 17:59 mkr_linux_386.tar.gz
-rw-r--r--  1 itchyny  staff   4.4M Mar 20 17:59 mkr_linux_amd64.tar.gz